Hardening. Hardening is a heat treatment which consists in heating of steel to
temperatures exceeding temperature of phase transformations, endurance at this
temperature and the subsequent cooling with a speed exceeding critical
minimum speed of cooling. Main objective of hardening is obtaining high
hardness.
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Depending on temperature of heating distinguish:
- full hardening at which heating is carried out in single-phase austenitic area
(on 30-50z higher than the A3 line). At fast cooling there is a full transformation
of austenite into marten site.
- incomplete hardening at which heating is carried out in two-phase area
(between the A3 and A1 lines).
In practice full hardening apply for proeutectoid steel, incomplete for
hypereutectoid steel.
Heating temperature under training alloyed steel usually above, than for the
carbonaceous. Diffusive processes in alloyed steel proceed more slowly
therefore they require longer endurance.
For achievement of the maximum hardness when training seek to receive
marten site structure. The minimum speed of cooling necessary for overcooling
of austenite before marten site transformation, is called as the critical speed of
hardening. Speed of cooling is defined by a type of the cooling environment.
Usually for hardening is used boiling liquids:
- water.
- water solutions of salts and alkalis.
- oils.
The choice of the concrete hardening environment is defined by a type of a
product. For example, water with a temperature 18-25C use generally when
hardening details of a simple form and the small sizes executed from
carbonaceous steel. Details of more difficult form from carbonaceous and
alloyed steel temper in oils. For training alloyed steel often use water solutions.
Important characteristics steel, necessary for purpose of technological modes
of training, harden ability and a hardness penetration.
Harden ability characterizes ability of steel to hardness increase when
hardening depends mainly on the content of carbon in steel. Harden ability
estimate on the hardness of a blanket of a steel sample after training.
The hardness penetration characterizes ability began to be tempered on the
demanded depth. The hardness penetration is estimated on distance from a
product surface to a layer which contains not less than 50% of marten site.
Ways of hardening steel:
- hardening in one cooler at which the heated detail plunges into cooling liquid
and remains there before full cooling.
The simplest way. Shortcoming - emergence of considerable internal tension.
- hardening in two environments at which a detail to 300-400zs cool in water,
and then transfer to oil. Apply to reduction of internal tension at heat treatment
of products from tool high carbon steels. Shortcoming - difficulty of regulation
of endurance of details in the first environment.
- step hardening at which the detail is quickly cooled with immersion in a salt
bath with a temperature which is a little exceeding temperature of marten site
transformation, is maintained before achievement of identical temperature on all
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section, and then cooled on air. Slow cooling on air reduces the internal tension
and possibility of a buckling. Shortcoming - restriction of the size of details.
- isothermal hardening at which the detail is maintained in a salt bath before the
end of isothermal transformation of austenite.
- hardening with self-drawback at which in the hardening environment cool only